DescriptionThe effect of pre-existing mental illness on diagnosis/treatment delays and survival in elderly patients with breast cancer is poorly defined in the literature. Because early detection and treatment are critical to long-term survival, it is important to identify the role of pre-existing mental illness in diagnosis and treatment delays of breast cancer. Therefore, this study sought to (1) describe the demographic, clinical, and tumor characteristics of elderly patients with pre-existing mental illness that were also diagnosed with breast cancer; (2) compare diagnosis and treatment delays of elderly breast cancer patients with and without pre-existing mental illness, and (3) compare rates of all cause and breast cancer-specific survival for elderly patients with and without mental illness. A retrospective cohort study of women aged 68+ at breast cancer diagnosis (stage I, II, or IIIa) was conducted using the SEER-Medicare database. Descriptive statistics, logistic regression, binomial regression, Cox proportional Hazards regression and competing risk models were utilized for the analysis. Compared to those without mental illness, those with mental illness were slightly more likely to be white, reside in neighborhoods with a lower income, use tobacco, and have more medical comorbidities. Breast cancer patients with pre-existing mental illness were also less likely to receive mammography and to be married. Compared to those without mental illness, a higher proportion of patients with mental illness had poorly differentiated tumors and more positive lymph nodes. Breast-conserving surgery was more common than mastectomies overall, but patients with mental illness were less likely to receive adjuvant radiation following this procedure compared to those without mental illness. There were no significant differences in the risk of diagnosis, initial treatment, surgery, or adjuvant radiation treatment delays of ≥60 days or ≥90 days for those with versus without mental illness. However, breast cancer patients with pre-existing mental illness experienced a significant 13% higher risk of adjuvant chemotherapy delay of ≥90 days. When examining these associations by type of mental illness, patients with a diagnosis of both anxiety and depression were more likely to experience an increased risk of a diagnosis delay of ≥90 days. Breast cancer patients with major psychiatric illness (i.e., bipolar disorder, schizophrenia, or other psychotic disorders) were more likely to experience a prolonged initial treatment delay of ≥60 days. Patients with mental illness compared to those without faced a significant 43% increase in the rate of all-cause mortality and a non-significant 12% increase in the rate of breast cancer-specific mortality. The increased mortality from all causes remained for each category of mental illness, but the strongest impact was seen in those with major psychiatric illness (127% increased rate of all-cause death). Although the presence of mental illness overall did not significantly impact the rate of breast cancer death, a significant 40% increased rate was observed for patients with major psychiatric illness compared to those without mental illness. The findings of this dissertation provide evidence that breast cancer patients with major psychiatric illness experience significant treatment delays and face shorter survival. Breast cancer patients with major psychiatric illness would benefit from increased coordination of care among psychiatrists, primary care physicians, and other specialties. The reason for the increased breast cancer death rate in patients with pre-existing major psychiatric disorders deserves attention in order to understand the underlying mechanism(s).